Fusiact-B contains two active ingredients, betamethasone and fusidic acid. Betamethasone is a type of medicine called a topical corticosteroid. Corticosteroids are medicines used for reducing inflammation. Skin inflammation in eczema occurs due to the irritation of the skin, and is caused by the release of various substances involved in the immune system. These substances cause blood vessels to widen, resulting in the affected area becoming red, swollen, itchy and painful.